Method of fabricating a heat pipe

ABSTRACT

A tube is provided having a plurality of closely spaced very thin longitudinal fins projecting into its interior. The tube is particularly useful as a heat pipe, the capillaries defined between adjacent fins serving as a wicking structure for conveying a heat transfer working fluid from one region of the pipe to another by capillary pumping. Fabrication of the internally finned tube is accomplished by a sacrificial mandrel method.

United States Patent Werner et al.

[ May 30, 11972 Filed:

METHOD OF FABRICATING A HEAT PIPE Inventors: Richard W. Werner, Danville; Everette E. Alexander, Dublin; Irving J. Comstock, Livermore, all of Calif.

Assignee: The United States of America as represented by the United States Atomic Energy Commission May 18, 1970 App]. No.: 38,219

US. Cl. ..29/157.3 B, 29/ 157.3 R, 29/423, 29/460, 165/105 Int. Cl. ..B21d 53/02, B23p 15/26 Field of Search ..29/157.3 A, 157.3 B, 157.3 AH, 29/1573 R, 423, 460; 165/105 References Cited UNITED STATES PATENTS 3/1970 Levedahl ..29/157.3 R X 3,105,285 10/1963 Favre ..29/423 X 2,944,338 7/1960 Craig ..29/458 X 3,192,609 7/1965 Masatake Murata et a]. ..29/460 X 3,364,548 l/i968 Marco ..29/l57.3 R

3,314,130 4/1967 Sheridan ..29/423 3,402,767 9/1968 Bohdansky et al ..165/105 Primary Examiner-John F. Campbell Assistant Examiner-Donald C. Reiley, III Att0meyRoland A. Anderson [5 7] ABSTRACT A tube is provided having a plurality of closely spaced very thin longitudinal fins projecting into its interior. The tube is particularly useful as a heat pipe, the capillaries defined between adjacent fins serving as a wicking structure for conveying a heat transfer working fluid from one region of the pipe to another by capillary pumping. Fabrication of the internally finned tube is accomplished by a sacrificial mandrel method.

6 Claims, 5 Drawing Figures PATENTEDMAY 30 19?:

DEPOSITION PROCESS REMOVAL PROCESS SACRIFICIAL MATERIAL 8. r d C R 8 n 0 "at! mwm N m E .A VWE. N J I r g m m w RE BY MW ATTORNEY.

BACKGROUND OF THE INVENTION The invention disclosed herein was evolved under, or in the course of Contract No. W-7405-ENG-48 with the United States Atomic Energy Commission.

Heat pipes are advantageously employed in nuclear space reactor systems, and elsewhere, as heat transfer mechanisms and to radiate excess heat from the systems. In this regard a heat pipe is a self-contained thermal conductance device devoid of moving parts which transfers heat as latent energy by evaporating a working fluid in a heating zone and condensing the vapor thus produced in a cooling zone. Such a heat pipe is typically provided as a tubular container having a wicking structure extending along its inner peripheral surface for conveying the working fluid between longitudinally displaced condenser and evaporator zones thereof. Heat is absorbed in the fluid at the evaporator zone in the form of latent heat of vaporization and rejected at the condenser zone by condensation of the resulting vapor. The. fluid is returned to the evaporator zone via the wicking structure by capillary pumping action.

Here'tofore, heat pipe wicking structure has been typically provided as an annular roll of fine wire mesh inserted into a suitable tube and saturated with the working fluid. The capillary pumping obtainable therewith has been limited. Altematively, wicking structure has taken the form of parallel longitudinal grooves formed in the interior wall of the pipe, or a tubular insert coaxially disposed within the pipe. The grooves function as capillaries for returning the condensed working fluid to the evaporator zone. However, the capillary pumping action has been limited by the number or density of capillary grooves that can be provided on the interior of a pipe of small diameter by existing forming methods. In this regard it is possible to mill grooves in the inner wall of the pipe if there is sufficient interior space to permit the necessary manipulation of the milling head. Consequently in instances where the pipe inner diameter is quite small, for example less than 1 centimeter, milling of the grooves from the inside of the pipe becomes difficult, if not impossible. I-Ieretofore, the forming of internal grooves on such very small diameter pipes has involved pressing the pipe into an internally disposed longitudinally fluted profiling mandrel which may be chemically dissolved subsequent to fonnation of the grooves. With either forming method applied to a small diameter pipe of the order of l centimeter inner diameter the density of internal capillary grooves has been limited to not more than 60 per inch of circumference.

SUMMARY OF THE INVENTION The general object of the present invention is to provide heat pipe wicking structure of improved construction capable of increased capillary pumping of working fluid. More particularly, in accordance with the present invention there is provided an internally finned tube particularly useful as a heat pipe, the tube having a small inner diameter which may be of the order of l centimeter or less and having a plurality of very closely spaced, interiorly projecting very thin longitudinal fins defining a plurality of longitudinal capillary channels therebetween with a density of at least 100 per inch of inner tube circumference which serve as a highly efficient wicking structure.

Production of the tube is accomplished in accordance with a relatively simple sacrificial mandrel construction method which generally comprises the placing of long thin strips of tube material and spacer strips of sacrificial material on edge alternately about the outside surface of a sacrificial mandrel cylinder, depositing a layer of tube material to the peripheral surface of the resulting strip and mandrel structure, and disposing of the sacrificial mandrel cylinder and spacer strips whereby the internally finned tube remains.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a thin-walled cylindrical mandrel employed in a method of fabricating an internally finned tube in accordance with the present invention.

FIG. 2 is a perspective view, partially in schematic, depicting an assembly of the mandrel and strips of fin and spacer material and the manner in which it is employed during the initial stages of the internally finned tube fabricating method.

FIG. 3 is a broken out enlarged view of a portion of the assembly of FIG. 2.

FIG. 4 is a view similar to FIG. 2, but depicting an intermediate structure that is evolved and the manner in which it is employed during the latter stages of the method.

FIG. 5 is a perspective view of the internally finned tube resulting from the method of the invention, embodied as a heat pipe.

DETAILED DESCRIPTION OF THE INVENTION Considering now the invention in greater detail with reference to the drawings, there is provided an internally finned tube 1 l, of the type shown in FIG. 5. More particularly, the tube comprises an elongated hollow cylindrical body 12 having a plurality of closely circumferentially spaced, longitudinally extending very thin fins l3 projecting radially inward therefrom. The density of the fins is extremely high, being representatively of the order I00 fins per inch of interior surface of the body. In addition, the inner diameter of the body may be quite small, for example from a fraction of a centimeter to about a centimeter. The spacing between fins may be equal to fin thickness.

By virtue of the high density of fins provided in a small diameter tube with spacing between fins equal to fin thickness, an equal very large number of capillary channels are created between fins which thereby render the tube of the present invention particularly useful in heat pipe applications. In this regard, a heat pipe is provided by introducing a suitable working fluid, such as liquid sodium, into a length of the tube 11 and sealing the opposite ends thereof, as by means of closures l4 hermetically secured thereto. The high density array of thin fins thus extends longitudinally between the opposite end closures l4 anddefines a very large number of capillary channels which serve as a highly efficient wicking structure for the working fluid. The capillary pumping of the wick structure can be further improved by the addition of a single layer of fine wire mesh on top of the thin fins. Heat applied to an evaporator region of the pipe thereof is absorbed by the working fluid in the form of latent heat of vaporization. The resulting vapor transfers the heat to a condenser region of the pipe where the heat is rejected by condensation. The condensed fluid is returned to the evaporator region via the finned wicking structure by capillary pumping action.

As a specific example of heat pipe wicking structure in accordance with the present invention, the tube may have an inner diameter of 1 centimeter while the fins are 0.005 inch thick with a space between fins of 0.005 inch, and the fins projecting radially inward 0.04 inch. As a result, there are 123 capillary channels defined between the fins, which is about twice the number of longitudinal capillaries that have been provided in existing heat pipes of comparable internal diameter.

It will be appreciated that it is extremely difficult, if not impossible, to fabricate the internally finned tube 11 of the present invention utilizing conventional manufacturing techniques by virtue of the high density and close spacing of very thin fins and the small tube diameters involved. Accordingly, the present invention also provides a novel method of fabricating the tube 11, the steps of which method are depicted in FIGS. 14. In accordance with the method there is initially provided an elongated cylindrical sacrificial mandrel 16, preferably a thin walled tube as shown in FIG. 1.

As a particularly salient step of the method, a plurality of elongated rectangular fin defining thin foil strips 13 of a desired tube material and elongated rectangular spacer strips 19 of mandrel, or other sacrificial material, are alternately placed on edge about the periphery of the mandrel parallel to the longitudinal axis thereof. The resulting assembly 21 of mandrel and strips of fin and spacer material is as shown in FIGS. 2 and 3. In the formation of the assembly, the mandrel may, for example, be fitted with radially slotted wheels (not shown) at each end. Each slot has a width substantially equal to the thickness of two foil strips 13 and one spacer strip 19. The width of each land between adjacent slots is substantially equal to the thickness of a spacer strip. The slot depth is such as to terminate at the periphery of the mandrel. A sandwich of two fin strips and an interposed spacer strip is then fitted into each aligned pair of slots of the wheels at the opposite ends of the mandrel such that the inner edges of the strips contact the mandrel periphery. Slightly shortened spacer strips are fitted into the spaces between adjacent strip sandwiches opposite the wheel lands, whereby an alternate succession of the fin and spacer strips are supported in contact with the mandrel about its periphery. The structural integrity of the assembly may be enhanced by tying the strips in place with circumscribing loops of wire adjacent the opposite ends of the mandrel.

A layer of tube material is next deposited upon the periphery of the assembly 21, in bonded relation thereto, to a desired thickness to form the tube body 12. This is accomplished by subjecting the assembly to a deposition process, as depicted by functional block 22. In this regard, a vapor deposition, electrolytic deposition, or equivalent process may be employed. As a result there is provided an intermediate structure 23, as shown in FIG. 4, which centrally includes the mandrel l6, thin fin strips 13 and spacer strips 19 alternately radially projecting from the mandrel periphery, and deposited annular layer of tube material circumscribing the strips and defining the tube body 12. I

The mandrel 16 and spacer strips 19 are then removed from the intermediate structure 23 by subjecting same to a sacrifi cial material removal process, as indicated by functional block 26. The removal process may, for example, consist of subjecting the structure to a selective dissolution agent, such as a bath of acid or base, which is effective to dissolve the mandrel and spacer strip materials, but not the tube material. Alternatively, the mandrel and spacer strip materials may be selected to have a lower melting temperature than the tube material whereby the structure 23 may be heated to a temperature which effects melting of the mandrel and spacer strips, but not the fins and circumscribing annular layer of tube material. It will be appreciated that by virtue of a thin walled tube being employed as the mandrel, the amount of mandrel material to be removed is minimized. Subsequent to removal of the sacrificial material there remains an internally finned tube 11 in accordance with the present invention, as shown in FIG. 5, wherein the body 12 is defined by the deposited layer of tube material and the fins are defined by the foil strips 13.

Considering now several specific examples of the tube 11 with reference to the previously described method of fabricating same, a tube that is particularly advantageous for heat pipe applications is fabricated of tungsten. In this case the mandrel 16 is preferably of molybdenum as are the spacer strips 19, while the fin strips 13 are of tungsten in the formation of assembly 21. In the deposition process 22, tungsten is vapor deposited upon the periphery of the assembly 21 to form the tube wall or body 12 and thereby provide the intermediate structure 23. The sacrificial material removal process 26 then involves placing the structure 23 in a bath of hydrofluoric acid which dissolves the molybdenum mandrel and spacer strips and leaves the internally finned tube of tungsten.

An internally finned tube of copper may be advantageously fabricated in an analogous manner. In this case the assembly 21 is constructed of an aluminum mandrel 16, aluminum spacer strips 19, and copper fin strips 13. In accordance with the deposition process 22, copper is deposited upon the periphery of the assembly 21 to thereby produce the structure 23 wherein the tube wall 12 is of copper. The sacrificial material removal process 26 then advantageously comprises placing the structure 23 in a bath of sodium hydroxide which is effective to dissolve the aluminum mandrel and spacer strips while leaving the internally finned copper tube.

In the fabrication of the previously mentioned heat pipe tube having 123 capillary channels for an inner diameter of 1 centimeter, the fin defining strips 13 and spacer strips 19 are placed on the mandrel 16 to provide the assembly 21 with a diameter A of 1 centimeter. The mandrel wall thickness B may be 0.04 inch. The thickness C of spacer strips 19 and thickness E of fin strips 13 are both 0.005 inch, and the width D of such strips is 0.04 inch. A 0.04 inch layer of material is deposited on the periphery of the assembly 21 to form the tube wall 12 and the mandrel and spacer strips are dissolved, thereby providing the specified internally finned tube.

What is claimed is:

1. A method of fabricating a heat pipe comprising an internally finned tube of a desired tube material having a plurality of closely circumferentially spaced longitudinally extending very thin fins projecting radially inward therefrom, said fins defining a plurality of longitudinal capillary channels therebetween, said method comprising the steps of placing a plurality of elongated rectangular thin foil strips of said desired tube material and elongated rectangular sacrificial spacer strips alternately on edge about the periphery of a cylindrical sacrificial mandrel parallel to the longitudinal axis thereof, depositing an outer layer of said desired tube material to the peripheral edges of said strips in circumscribing bonded relation thereto to form an intermediate structure, and removing said sacrificial spacer strips and mandrel from said intermediate structure to thereby leave an internally finned tube of said desired tube material wherein said outer layer defines a hollow cylindrical tube body and said thin strips of said desired tube material define fins projecting radially inward from the body.

2. A method according to claim 1, further defined by said step of removing said sacrificial spacer strips and mandrel comprising subjecting said intermediate structure to a selective dissolution agent effective to dissolve said spacer strips and mandrel while leaving said desired tube material.

3. A method according to claim 2, further defined by said desired tube material being tungsten and said spacer strips and mandrel being of molybdenum, and said step of depositing an outer layer comprising vapor depositing tungsten upon the periphery of said strips to a desired thickness to form said outer layer in circumscribing relation thereto.

4. A method according to claim 3, further defined by said step of subjecting said intermediate structure to a selective dissolution agent comprising placing said intermediate structure in a bath of hydrofluoric acid.

5. A method according to claim 2, further defined by said desired tube material being copper and said spacer strips and mandrel being of aluminum, and said step of depositing an outer layer comprising vapor depositing copper upon the periphery of said strips to a desired thickness to form said outer layer in circumscribing relation thereto.

6. A method according to claim 5, further defined by said step of subjecting said intermediate structure to a selective dissolution agent comprising placing said intermediate structure in a bath of sodium hydroxide. 

1. A method of fabricating a heat pipe comprising an internally finned tube of a desired tube material having a plurality of closely circumferentially spaced longitudinally extending very thin fins projecting radially inward therefrom, said fins defining a plurality of longitudinal capillary channels therebetween, said method comprising the steps of placing a plurality of elongated rectangular thin foil strips of said desired tube material and elongated rectangular sacrificial spacer strips alternately on edge about the periphery of a cylindrical sacrificial mandrel parallel to the longitudinal axis thereof, depositing an outer layer of said desired tube material to the peripheral edges of said strips in circumscribing bonded relation thereto to form an intermediate structure, and removing said sacrificial spacer strips and mandrel from said intermediate structure to thereby leave an internally finned tube of said desired tube material wherein said outer layer defines a hollow cylindrical tube body and said thin strips of said desired tube material define fins projecting radially inward from the body.
 2. A method according to claim 1, further defined by said step of removing said sacrificial spacer strips and mandrel compriSing subjecting said intermediate structure to a selective dissolution agent effective to dissolve said spacer strips and mandrel while leaving said desired tube material.
 3. A method according to claim 2, further defined by said desired tube material being tungsten and said spacer strips and mandrel being of molybdenum, and said step of depositing an outer layer comprising vapor depositing tungsten upon the periphery of said strips to a desired thickness to form said outer layer in circumscribing relation thereto.
 4. A method according to claim 3, further defined by said step of subjecting said intermediate structure to a selective dissolution agent comprising placing said intermediate structure in a bath of hydrofluoric acid.
 5. A method according to claim 2, further defined by said desired tube material being copper and said spacer strips and mandrel being of aluminum, and said step of depositing an outer layer comprising vapor depositing copper upon the periphery of said strips to a desired thickness to form said outer layer in circumscribing relation thereto.
 6. A method according to claim 5, further defined by said step of subjecting said intermediate structure to a selective dissolution agent comprising placing said intermediate structure in a bath of sodium hydroxide. 